The present invention relates to a throttle control apparatus for an internal combustion engine, and more particularly to such a throttle control apparatus that controls the output power of the engine by means of an electronically-controlled actuator such as a motor.
In general, the amount of intake air sucked into an engine of an automotive vehicle is controlled by the opening and closing of a throttle valve which is disposed in an intake passage or pipe. The throttle valve is usually mechanically connected through a wire cable with an accelerator pedal so that it is opened and closed by the accelerator pedal in a mechanical fashion.
In recent years, however, in order to improve the driving feeling of an operator and the running performance of an automobile, many throttle control apparatuses have been studied and developed in which a throttle valve is operatively connected with an accelerator pedal through a wire cable with an electronic control means interposed therein so that it is operated to open and close under the action of an electronically-controlled acutator which is controlled based on an electric signal corresponding to a control amount of the throttle valve (i.e., the amount of rotation of the throttle valve required to get a target throttle opening degree) which is successively calculated based on an accelerator pedal signal representative of the amount of depression of the accelerator pedal operated by the operator, and other signals indicating the operating conditions of the engine and/or the running conditions of the vehicle such as, for example, an engine speed signal representative of the number of revolutions per minute of the engine, a gear position signal representative of a shift gear position, a wheel slip signal representative of slip of the vehicle's wheels, etc.
With this electrically-controlled type of throttle control apparatus, however, if the electronically-controlled actuator or the electronic controller for operating the throttle valve fails, there is a possibility that the throttle valve is continuously held open irrespective of the operator's desire, causing the vehicle to accelerate. Therefore, it is necessary to provide a safety or failsafe device for avoiding such a dangerous situation.
In the past, Japanese Patent Publication No. 61-54933 disclosed an electronically-controlled throttle control apparatus which has a first throttle valve adapted to be operated by an electronically-controlled actuator and a second throttle valve disposed in serial relation with the first throttle valve, the second throttle valve being operatively connected with an accelerator pedal so that it acts as a safety means in case of a failure of the electronically-operated actuator in which the first throttle valve is held open.
Also, Japanese Patent Laid-Open No. 61-60331 discloses another type of electronically-controlled throttle control apparatus which has an accelerator-pedal-operated first throttle valve and an electrically-operated second throttle valve both serially disposed in an intake passage of an engine. The second throttle valve is electrically operated to open and close for slip control during acceleration, whereas the first throttle valve can be operated to open and close under the action of an accelerator pedal in order to secure safety during driving in cases where the second throttle valve fails.
In both of the above-described conventional throttle control apparatuses having an electronically-controlled throttle valve and an accelerator-pedal-operated throttle valve, it is possible to prevent a run-away condition of a vehicle and thus secure driving safety under the action of the second throttle valve operatively connected with the accellerator pedal even if the first throttle valve operated by an electrically-controlled actuator fails. However, to dispose two throttle valves in the intake passage in a serial relation with each other enlarges the structure of the entire intake system with the result that difficulty arises in installing such a large intake system in a relatively limited engine compartment of a vehicle. Further, neither of the above-described conventional throttle control apparatuses can be applied to a vehicle which has a cruise control device for enabling the vehicle to automatically cruise without the accelerator pedal being operated by the operator since in both of the above conventional apparatuses, it is required to operate an accelerator pedal in order to open and close the electrically-operated throttle valve.